Semiconductor devices are very small, typically from 5 mm×5 mm square to 50 mm×50 mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder ball/bump or copper pillar or wire.
It is necessary to test the bond strength of the bonds, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately.
There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling the bond away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond.
Machines that perform these tests typically comprise a bond test tool, be it a shear test tool, push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool are moved in order to perform the test, which comprises measuring the force needed to break the bond. Each bond test tool is mounted to a load cell provided with a force transducer and associated electronics.
While there are a variety of different test tools, the part of the machine used to position the test tool and move the test tool or bond during a test may be the same for each test tool. Accordingly, test tools have been designed to be removable from the rest of the machine so that they can be replaced with a different type of test tool or a test tool suitable for a different range of measurements or a different type of bond or test. It may be desirable to use several different test tools on a single substrate under test and so an operator may be required to perform replacement of test tools frequently.
When performing a bond test, it is necessary to illuminate the bond under test with sufficient light to permit accurate pre-alignment and set-up of the test, video recording and visual inspection of the test as it occurs, and subsequent visible inspection of the bond and test tool once it is complete.
Bond test machines are typically fitted with light stalks, which are flexible stalks with a light fitting on a free end. These are fixed to the bond test machine and can be manoeuvred into position to illuminate the test piece. However providing illumination in this way has a number of problems. The light stalks are bulky, and can interference with other elements of bond test machine operation. The position of the light sources inevitably means that there is shadow and inconsistent illumination. The light stalk may be moved accidently and may therefore require frequent re-adjustment. The remote position of the light sources means that illumination may not be optimised for use with miniature test pieces which are now becoming more commonplace (miniature in this context means in the size range of 20 to 100 μm (microns)).
It is an object of the invention to address these problems or at least to provide a useful alternative solution to the problem of illumination in a bond test machine.